Update to guacamole 1.6.0

[helendduncan]

✅ Checklist

• I have searched open and closed issues for duplicates.
• This is a request for a new feature in the Data Safe Haven or an upgrade to an existing feature.
• The feature is still missing in the latest version.
• I have read through the documentation.
• This isn't an open-ended question (open a discussion if it is).

🍓 Suggested change

Update to the newly-released guacamole

🚂 How could this be done?

[cptanalatriste]

This was implemented as part of this PR #2453 . It should be present in the next release.